Thursday, December 5, 2013

Resilient Structures

When in doubt, Make it stout,
of stuff you know a lot about.  -Structural Engineer's Motto

Exotic materials have exotic failure modes.  -Structural Engineer's Creed

Resilient structures share some common attributes.

Failure or overload of an individual member does not result in catastrophic failure of the structure.  The overloaded member continues to carry load, even when "over capacity."  The structure is able to recruit alternate load paths and spill load from the overloaded member to the alternate paths. 

...continues to carry load

Brittle materials fracture when overloaded and are not capable of carrying load because the member separated into two or more a broken pane of glass.

Ductile materials stretch and work harden.  Common structural steel, brass, 300 series stainless steel are all ductile materials.

Some geometries become more capable with load.  
-A sail stiffens as it fills with wind while a thin rod collapses in compression.  
 -A thin walled tube (picture an aluminum beverage can) will collapse in bending because it will lose its depth of section.  The technical term is that "the section crippled".  Honest.  A thicker walled tube retains the integrity of its original shape longer than the thin wall section and will carry load and absorb energy longer into an event.


The most cost efficient structure is a "statically determinate" truss.  Each element is in pure tension or compression.  Failure of one strut or joint in a truss will cause the entire structure to collapse because there is no mechanism to "spill" the load carried by that element to any other element.

The most cost efficient structure has every element stressed at the mathematical maximum allowed.  There is little room for noise or failures of imagination.

The most cost efficient structure is made from higher strength, lower ductility material.

The most cost efficient structures tend to be fabricated with straight lines, square corners and welds.

Liberty ships were cost fabricate.

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